Jammah is located between Mussannah and Rustaq, about an hour and a half from Muscat. The turnoff point coming from Mussanah is at the same roundabout where one turns for Al Hamn Castle, but instead of turning right (for Al Hazm Castle), you turn left to reach Jamma. It is 6 kms from Mansur Castle (which I never managed to find!). If you look at the map here, Jamma is indicated with a red circle.
Andy in Oman 